The present invention relates to a bending machine for sheet metal panels having a blank holder with uniform compression.
Several bending machines are known which have a fixed counterblade on which a sheet metal panel is positioned for bending. The latter is held by a blank holder while a blade consisting of a pair of counterblades executes the required bend of the edge of said sheet metal panel.
An example of such a bending machine is described in the Austrian Pat. No. 372883 in the name of Voest Alpine Aktiengesellschaft.
In such a bending machine a blank holder slides in lateral guides which are integral with the base of the machine to rise up and come down on a sheet metal panel positioned on a fixed counterblade. The blank holder is operated from above by hydraulic cylinders placed at its ends and is integral with arc-shaped supporting arms, on which actuators for moving the two blades are hinged.
The fixed counterblade is in its turn integral with the fixed base of the machine, on which additional actuators for moving the blades are hinged.
The greatest drawback which occurs in a bending machine of this type is that the forces which urge the blank holder upwards during the bending operation are not uniformly distributed in the direction of the line of bending but are concentrated more or less along the centre line of the machine according to the length of the bend to be executed and to the thickness of the sheet metal panel itself. It follows that the distance between the active edge of the blank holder and the underlying counterblade is greater in the centre than at the outer ends, so that, since the bending line follows the line of the blank holder's active edge, the bend is not straight. For the same reason it is not possible to make a so-called "safe hands" uncrushed U-shaped bend, because the distance between the superimposed edges would be less at the ends than at the centre.
The problem is certainly not solved by the two cylinders which press the blank holder downwards. Since the two cylinders are at the ends of the blank holder, they are not in a position to generate uniformly distributed forces, except with a hypothetical infinitely rigid blank holder. In fact, if the sheet metal to be bent is short and of substantial thickness, such a drawback is heightened; the blank holder actually tends to bend inwards by pivoting on the edges of the sheet metal, so that the central part of the blank holder rises up and assumes a concave configuration.
To overcome this drawback it is possible to exert pressure on the blank holder with a multiplicity of hydraulic cylinders uniformly distributed on the upper part of the machine, so as to counteract the force which tends to bend it, but this solution is of little advantage since it increases both cost and weight of the known bending machine and also makes its structure more complicated.